When a signal passes through a network, the network adds noise to the signal. Because noise limits the amount of information a signal having a given power can contain, noise minimization is often desired in limited signal power situations, for example, satellite communications.
A standard measure for the amount of noise added to a signal by a two-port network is termed "noise factor" ("noise figure" when measured in decibels), and is defined in "IRE Standards on Methods of Measuring Noise in Linear Twoports, 1959," 59 IRE 20. Sl, Proc. IRE, vol. 48, pp. 60-68, January 1960, reprinted in Low-Noise Microwave Transistor & Amplifiers IEEE Press, 1981:
"The noise factor, at a specified input frequency, is defined as the ratio of (1) the total noise power per unit bandwidth at a corresponding output frequency available at the output port when the noise temperature of the input termination is standard (290 K) to (2) that portion of (1) engendered at the input frequency by the input termination."
The noise factor F of a two-port network is a function of the source admittance Y.sub.s and varies according to the equation ##EQU1## F.sub.0 is the network's "optimum noise factor" and is the minimum noise factor which may be achieved by setting the source admittance Y.sub.s to an optimum admittance Y.sub.0. R.sub.n is a parameter having the units of resistance, and G.sub.s is the real or conductance component of the complex source admittance Y.sub.s, the imaginary component B.sub.s being susceptance. Since Y.sub.s =G.sub.s +jB.sub.s and Y.sub.0 =G.sub.0 +jB.sub.0, equation (1) may be rewritten as ##EQU2## A network's noise performance at a given frequency can be described by specifying its four noise parameters F.sub.0, G.sub.0, B.sub.0, and R.sub.n. These four parameters are independent of the source and load terminations. A graph of noise factor F on the rectangular source admittance plane is shown in FIG. 1.
Noise factor F also may be graphed on the complex reflective coefficient plane, i.e., a Smith chart, as described by H. Fukui in "Available Power Gain, Noise Figure, and Noise Measure of Two-Ports and Their Graphical Representation," IEEE Trans. Circuit Theory, vol. CT-13, pp. 137-142, June 1966, and also reprinted in Low-Noise Microwave Transistors & Amplifiers. A Smith chart having circles representing source reflection coefficients which yield constant noise figures for a hypothetical DUT is shown in FIG. 2.
A DUT's noise parameters for a given frequency can be calculated from the DUT's noise factor at the same frequency for four different source admittances. Due to inevitable errors in measurement and possible singular solutions, preferably more than four measurements are made and the noise parameters are calculated using a fitting method, one method being described by Richard Q. Lane in "The Determination of Device Noise Parameters," Proc. IEEE, vol. 57, pp. 1461-1462, August 1969 which is also reprinted in Low-Noise Microwave Transistor & Amplifiers.
To assure accurate calculation of the noise parameters, and especially the minimum noise factor F.sub.0, some noise factor measurements should be taken which are not on the steep slope of the surface F shown in FIG. 1. To ensure this, the noise power should be measured for many different source admittances, or source reflection coefficients, such that the measurement points permeate the entire Smith chart. Such a pattern of measurement points on a Smith chart will be called hereinafter a "constellation."
In Leake, U.S. Pat. No. 4,502,208 an electrically controllable tuner is described. However, the tuner does not provide a constellation which fills the Smith chart; the points are concentrated in the center. Vahe Adamian and Arthur Uhlir, Jr. describe using off-set short circuits as the source admittance. ("Simplified Noise Evaluation of Microwave Receivers," IEEE Transactions on Instrumentation and Measurement, Vol. IM-33, No. 2, June 1984.) Unfortunately, such tuners only allow constellations with points on the edge of the Smith chart.
Larock and Meys describe a method of changing the source admittance in noise measurements by changing the measurement frequency a relatively small amount. ("Automatic Noise Temperature Measurement Through Frequency Variation," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-30, No. 8, August 1982.) A slight change in measurement frequency causes the measurement source admittance points to move in an arc around the center point of a Smith chart. With relatively small changes in measurement frequency, a measurement source admittance point may be moved full circle around the Smith chart.
Since noise parameters vary according to signal frequency, a complete noise parameter analysis requires multiple measurements of noise factor at multiple frequencies. Since noise is a random event, an accurate measurement of its power requires increasing time for decreasing bandwidth of measurement. Thus, accurate measurements of noise parameters can require significant time.
What is needed, then, is an improved method and apparatus for accurately measuring and calculating noise parameters of a two-port network while decreasing the time required for the measurements. Also needed is a method and apparatus for providing a broad constellation of source admittances, yielding more useful measurements of noise factor from which to calculate a DUT's noise parameters.